1. Field of the Invention
The subject invention generally relates to computer generated holograms and, more particularly, to a method of reducing noise in diffraction optical elements made from computer generated hologram originals.
2. Description of Related Art
Many systems involve complex optical power, such as head-up displays, helmet mounted displays, and even automotive holographic displays. These systems require, or at least benefit from, high-grade optical elements which might be produced from computer generated holograms (CGH's). A general discussion of making a copy hologram from a master hologram is found in U.S. Pat. No. 4,530,564 and incorporated herein by reference. As is well known, in a CGH, the design of the desired wavefront is mathematically described and a computer can then form a two-dimensional amplitude hologram representative of that wavefront. The computer can drive a printer to produce the desired diffraction grating on a substrate or, alternatively, create the grating by electronic or chemical procedures.
It is anticipated that head-up displays (HUD's) will employ computer generated holograms rather than aspheric mirrors to provide one or both of the necessary wavefronts to construct the optical element. This might be particularly true in modern aircraft where there is frequently minimal space in the cockpits. This limitation requires that the optical system be folded and compressed for a HUD. Complex aspheric reflecting mirrors are therefore required. Aspheric mirrors have been fabricated for use as a master reference in producing HUD holograms. This approach, however, involves lengthy and expensive procedures of grinding an aspheric mirror to meet the requirements of a particular HUD. An aspheric mirror also has a limitation as to how close it can be placed relative to the recording medium for replicating the aspheric diffraction grating in the recording medium.
Holograms, such as CGH's, producing the same effect as the aspheric mirrors, represent a lightweight and efficient solution to the above. However, a major problem in using a CGH as an imaging source for the fabrication of a holographic HUD combiner is the noise or artifacts in the CGH. These artifacts are present in the form of general nonuniformities of brightness and in multiple order scattering. The multiple order artifact results from the fringe pattern being generally formed in abrupt discrete units rather than a sinusoidal variance, as in an ideal holographic recording of a laser interference pattern.
In the past, spatial filtering techniques have been used to remove some of these artifacts, such as small, abrupt, high frequency changes.
One spatial filtering method adapted for a CGH includes a relative motion system developed by the present inventors. In that motion system, multiple order artifacts are sought to be removed by using a moving point source of coherent illumination for illuminating the computer generated diffraction grating. By providing a slight movement to the point source, the desired recording ray is changed only slightly. However, the noise scattering rays, which are the result of individual scattering elements rather than the diffraction effect of the complete assemblage of the fringe pattern, are displaced by a substantial distance. As a result, the scatter holograms are reduced by being smeared out with minimal effect on the main hologram.
One major drawback to the prior art filtering techniques is the limited extent to which artifacts are filtered. For example, broad, low frequency variations may not be adequately removed, if at all. Another drawback is that the techniques tend to be cumbersome and time consuming.
A need therefore exists in the art to provide an improved method for reducing noise created by artifacts in computer generated holograms. A multiple intermediate hologram technique has been employed in the prior art but is used to provide more optical power in the final hologram. According to this method, each intermediate step includes a new computer generated hologram with its own set of computer generated artifacts. This, however, does not teach or suggest the use of intermediate holograms to remove artifacts according to the present invention.